1. Field of the Invention
The present invention relates to a semiconductor device for controlling rotation of a motor.
2. Description of the Related Art
A device using a motor (e.g., DC motor) includes a motor driving apparatus as shown in FIG. 1. FIG. 1 is a schematic diagram showing a motor driving apparatus 10. The motor driving apparatus 10 includes a motor 11, a sensor for detecting the rotational position and the rotational frequency (number of revolutions) of the motor 11, a semiconductor device 13 serving as a motor control IC for controlling rotational frequency and protection operation of the motor 11, and a driver 14 for supplying driving current for driving the motor 11. The motor driving apparatus 10 operates as a closed loop by repeating the processes of analyzing the rotational frequency and phase data of the motor 11 detected by the sensor 12 by using the semiconductor device 13 and correcting the rotational frequency and the phase of the motor 11 so that the rotational frequency and the phase are controlled to desired values.
By setting the overall gain of the closed loop of the motor driving apparatus 10 to an appropriate value, the motor 11 can be steadily rotated. As shown in FIG. 2, the overall gain of the closed loop of the motor driving apparatus 10 is determined according to the gain of its speed system (speed system gain), the gain of its phase system (phase system gain), the gain of its loop (loop gain), the gain of its drive system (drive system gain), and the characteristics of the motor 11. FIG. 2 is a schematic diagram for describing the overall gain of the motor driving apparatus 10.
As shown in FIG. 3, the speed system gain, the phase system gain, and the loop gain are each determined (fixed) according to the constant of a component that is externally mounted on the motor control IC (semiconductor device) 13. FIG. 3 is a schematic diagram for describing an external (outside) component of the motor driving apparatus 10. The driving gain is determined by controlling the electric current value of the drive current supplied to the driver 14. Therefore, in order to control the rotation of the motor 11 for the above-described motor driving apparatus 10, the overall gain of the closed loop is changed by adjusting only the drive gain by changing the drive current supplied to the driver 14.
For example, in a case of activating a motor of a motor driving apparatus such as the above-described motor driving apparatus 10, a large drive torque is required for rotating the stopped motor (stopped state of motor). In such a case, the motor driving apparatus 10 is controlled so as to increase follow-up speed and shorten the activation time by increasing drive current and increasing the drive gain.
For example, Japanese Laid-Open Patent Application No. 2005-198468 (hereinafter referred to as “Patent Document 1”), discloses a motor control apparatus and an image forming apparatus that reduces the time for starting a printing operation by shortening the time of waiting for a polygon motor to reach steady state rotation. Furthermore, Japanese Laid-Open Patent Application No. 2005-110368 (hereinafter referred to as “Patent Document 2”) discloses a motor driving apparatus, an integrated circuit, and a motor driving method that can switch the gain for outputting control signals.
However, with the above-described motor driving apparatuses, the change of speed of the motor becomes greater as the drive gain increases. This also increases unevenness in the rotation of the motor. Furthermore, the above-described motor driving apparatuses require a large drive current for increasing their drive gains. Therefore, a large drive current is supplied to the circuitry (circuit components) in the motor driving apparatuses. This leads to an increase of heat in the circuitry and deterioration of or damage to the circuitry.
In a case where the maximum value of the drive current is not appropriately set, the jitter (ringing) as shown in FIG. 4 does not settle out even after the rotational frequency of the motor reaches a desired rotational frequency, thereby requiring time for the drive current to stabilize. FIG. 4 is a schematic diagram for describing the start-up (activation) time and the rotational frequency of a motor of a related art case. Accordingly, an apparatus (e.g., image forming apparatus) having a motor driving apparatus of a related art case requires a long start-up time.
Furthermore, the above-described motor driving apparatuses control rotation by only changing the drive gain where the speed system gain, the phase system gain, and the loop gain remain fixed (unchanged). This lowers the versatility of the motor driving apparatus.